1. Field of the Invention
The present invention relates generally to a photodetector and a photo detecting system. More specifically, the invention relates to a photodetector and photo detecting system capable of detecting information about the two-dimensional intensity distribution and wavelength distribution of incident light by the improvement of a transmission photodetector.
2. Description of Related Art
Now that optical technology is applied to various systems, light receiving elements serving as light detecting means are applied as key devices in applications for all of light applied apparatuses. These light receiving elements are devices which are formed of a semiconductor and which generate an electromotive force by the effect of a p-n junction. The mechanism of the photo electric transfer of a light receiving element will be briefly described below.
The mechanism of the photoelectric transfer is a mechanism wherein electrons and positive holes finally accumulate in an n-region and a p-region, respectively, to cause a potential difference although behavior is different in three places, i.e., in the vicinity of the surface, in a space charge region and within a crystal. First, light beams absorbed into the vicinity of the surface excite a high concentration of electrons and positive holes to cause the electrons and positive holes to diffuse toward the interior at which the concentrations of the electrons and positive holes are low. When the electrons and positive holes reach the space charge region of the p-n junction, the electrons drop in accordance with the potential gradient to reach the n-region, and the positive holes are blocked by the potential gradient to stay in the inlet of the space charge region.
Then, light beams absorbed in to the p-n junction space charge region slightly inside of the surface produce electrons and positive holes there, and the electrons and positive holes move in accordance with the potential gradient to accumulate in the n-region and p-region. Finally, only the long-wavelength components of light beams reach the interior of the crystal, and the positive holes produced in the inside n-region reach the space charge region by diffusion to be collected in a p-region by the electric field. Thus, the electromotive force is generated on both sides of the p-n junction when the p-n junction is irradiated with light.
Conventional light receiving elements are generally arranged at the end of an optical path as shown in, e.g., FIG. 35, since light beams do not pass through the surface forming p-n junction. That is, in FIG. 35, light beams which are split by a beam splitter 2 provided in an optical path 1 are led to a light receiving element 3. Therefore, the detecting optical system having the light receiving element 3 often has a complicated construction. In addition, by providing the beam splitter 2, it is not possible to avoid shifting the optical axis and generating noises on the wave front of beams to have a bad influence. Therefore, particularly in measuring systems requiring to propagate beams at a long distance, there is a problem in that it is difficult to adapt such a detecting optical system.
On the other hand, with the advance of optical applied system in recent years, it is increasingly requested to propagate light beams of a plurality of wavelength on the same optical axis. Conventionally, systems for dispersing light beams using a half mirror or a polarized beam splitter taking account of the polarized state are often used for detecting light beams having a single wavelength such as laser beams. However, with the development of recent short-wavelength lasers, there are many examples where lasers of a plurality of wavelengths have the same optical axis. In an optical disc system serving as one of the examples, two lasers having different wavelengths of 780 nm and 635 nm are used for reading a compact disc (CD) and a digital versatile disc (such as DVD) by the same system. In the current technology, detecting systems for the two wavelengths are often separately constructed, so that it is considered that the construction of the detecting optical system is increasingly complicated with the advance of optical technology.
As one of examples where incident light has a wide frequency band, there is an image sensor. As a typical example, there is a charge coupled device (CCD) for video camera or the like wherein natural light is incident thereon. The image sensor for video camera is provided with an optical system having three CCDs, as shown in FIG. 36, for dividing incident light beams into three kinds of wavelength bands of RGB to detect the divided light beams, respectively. That is, in the construction shown in this figure, a red component is separated by a prism 5, which is arranged in an optical path 1, to be detected by a CCD 4A. In addition, a green component separated by a prism 6 is detected by a CCD 4B, and a blue component separated by a prism 7 is detected by a CCD 4C. By adapting this detecting optical system, a light-intensity signal detected after the separation into primary colors is extracted as an information signal. In order to realize a high performance image sensor, it is required to separate incident light of such a wide wavelength band into light beams of primary colors to provide detecting systems for the respective primary colors, and it is indispensable to provide the complicated detecting system shown in FIG. 36.
In order to simplify the above described detecting optical system, a photodetector for partially transmitting light beams is proposed. As conventional photodetectors having light transmitting functions, there are reverse transparent electrode optical sensors which are mainly used for a solar battery and which are provided with an amorphous silicon optical sensor on a glass substrate, and see-through optical sensors for allowing the transmission of light beams by forming a micropore in silicon.
However, in the reverse transparent electrode optical sensors, color is limited to the band gap of silicon. Therefore, colors other than red, e.g., blue and green, can not be absorbed so that the wavelength band is limited. In addition, since the see-through optical sensors allow the micropore to transmit light beams, it is not possible to enhance both of transmittance and conversion efficiency, and it is not possible to adjust absorption wavelength.
In addition, as shown in FIG. 37, a transmission photodetector is realized by producing a thin-film photodiode using a process for controlled-thinning a silicon substrate. That is, in the example shown in this figure, a silicon oxide mask M is utilized for controlled-thinning a part of a silicon substrate S by etching with TMAH (Tetra-Methyl-Ammonium-Hydroxide) (see FIG. 37(a)), a p-n junction is formed (see FIG. 37(b)), and electrodes E are formed to form a thin-film photodiode (see FIG. 37(c)).
However, in such a detector, there are problems in that the production costs are high and the light transmittance is low. In addition, since the spectra of wavelengths of light beams absorbed and detected are fixed due to silicon serving as the material, there is a problem in that the use as a photodetector is very limited.
As described above, the conventional photodetector having the light receiving element must be arranged at the end of the optical path since it cannot transmit lightbeams. Simultaneously, there are problems in that it is indispensable to disperse light beams by providing the beam splitter or the like in the optical path in order to lead light beams to the photodetector arranged at the end, that the construction of the optical system is complicated, that the optical axis is finely shifted, and that noises are generated on the wave front of beams.
With the advance of optical technology in recent years, if detecting optical systems, the number of which corresponds to the number of used wavelengths, are provided due to the use of lasers of a plurality of wavelengths and/or the use of the frequency band which is split to be detected, the detecting optical systems tend to be increasingly complicated.
In addition, although there are conventionally light transmitting detectors having limited uses, fine adjustment is difficult and the production costs are high, so that these detectors are not suitably used as inexpensive and high performance photodetectors.